1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic technology. In particular, the present invention relates to an image forming apparatus forming an image by charging and exposing a single-layer organic photosensitive drum to light.
2. Description of the Related Art
Conventionally, image forming apparatuses using electrophotographic technologies have been actively developed for use as electrostatic copiers or printers.
Briefly referring to FIG. 15, a conventional image forming apparatus 1 using an electrophotographic technology will be described. An image forming apparatus 1 includes a rotatable photosensitive drum 3 having a photosensitive layer 2 located on a surface thereof, a main charger 4 for uniformly supplying the photosensitive layer 2 with a prescribed level of electric charge, an optical device 5 for exposing the photosensitive layer 2 to light and forming an electrostatic latent image on the photosensitive layer 2, a developing device 6 for developing the electrostatic latent image formed on the photosensitive layer 2 into a toner image, a transfer device 8 for transferring the toner image on the photosensitive layer 2 onto a recording paper sheet 7, a cleaning device 9 provided with a cleaning blade for removing the residual toner on the photosensitive layer 2, and a charge removing lamp 10 for removing the residual charge on the photosensitive layer 2 and thus setting the surface potential of the photosensitive layer 2 at a prescribed uniform level. The main charger 4 includes a discharge wire 4b for performing corona discharge to the photosensitive film 2 and a sealed case 4a surrounding the discharge wire 4b and opened toward the photosensitive layer 2.
In the image forming apparatus 1 having the above-described structure, an image is formed in the following manner.
First, the main charger 4 uniformly supplies the photosensitive layer 2 with a prescribed level of electric charge. Next, light is radiated to the photosensitive layer 2 by the optical device 5, thereby forming an electrostatic latent image on the photosensitive layer 2. Then, toner is supplied to the photosensitive layer 2 by the developing device 6, thereby developing the electrostatic latent image into a toner image. The toner image on the photosensitive layer 2 is transferred to the recording paper sheet 7 by the transfer device 8. After the transference, the residual toner on the photosensitive layer 2 is removed by the cleaning device 9. Light is radiated on the photosensitive layer 2 by the charge removing lamp 10, thereby removing the residual charge on the photosensitive layer 2. Thus, the surface potential of the photosensitive layer 2 is uniformly set at a prescribed level. Thereafter, the photosensitive layer 2 is charged again by the main charger 4. Such a process is repeated in accordance with the rotation of the photosensitive drum 3.
It is known that a surface potential of the photosensitive layer 2 obtained by charging by the main charger 4 differs among different production lots because the electric characteristics of the photosensitive layer 2 of the photosensitive drum 3 differs among different production lots. In detail, the photosensitive layer 2 is exposed to light corresponding to a white area (non-image area) of a document after the surface potential of the photosensitive layer 2 is uniformly set. The surface potential of the photosensitive layer 2 corresponding to the white area obtained at a developing position differs among different production lots. Such non-uniformity in the surface potential of the photosensitive layer 2 causes a difference in the density of an image formed on the recording paper sheet 7 among different production lots.
In order to prevent such a problem, a light radiation apparatus C shown in FIG. 16 is conventionally used. The light radiation apparatus C includes a lamp B and a case A surrounding the lamp B and opened toward the photosensitive layer 2. Light is radiated by the light radiation apparatus C toward a charging area 24c of the photosensitive layer 2, thereby adjusting the surface potential of the photosensitive layer 2 so that the surface potential will be uniform even among different production lots. In general, where the surface potential of the photosensitive layer 2 provided by the main charger 4 is uniform, as the amount of light radiated by the light radiation apparatus C is larger, the surface potential of an area D irradiated by the light is lower. By adjusting the amount of light radiated by the light radiation apparatus C in accordance with the photosensitive characteristic of the photosensitive layer 2, the surface potential of the photosensitive layer 2 at the developing position can be uniform even among different production lots before the image on the photosensitive layer 2 is developed by the developing device 6.
However, the light radiation apparatus C does not function effectively when a different material is used for the photosensitive layer 2. The photosensitive layer 2 may be made of an inorganic photoconductive material, such as Se, or a single-layered or multiple-layered organic photoconductive material. The inventors of the present invention have found that the relationship between the amount of light radiated to the photosensitive layer 2 by the light radiation apparatus C and the surface potential thereof differs, depending on whether the photosensitive layer 2 is formed of an inorganic material or an organic material. FIG. 17 is a graph illustrating such relationship. Line L1 represents such a relationship obtained when the photosensitive layer 2 is formed of an inorganic material, and line L2 represents such a relationship obtained when the photosensitive layer 2 is formed of an organic material. As is appreciated from FIG. 17, where the amount of light radiated by the light radiation apparatus C is relatively small, reduction in the surface potential of the organic photosensitive layer (L2) is greater than such reduction of the inorganic photosensitive layer (L1).
Depending on whether an inorganic material or an organic material is used for the photosensitive layer 2, the amount of light radiated by the optical device 5 required for reducing the potential of the photosensitive layer 2 provided by the main charger 4 to surface potential SP1 is different. For example, such a light amount is E1 in the case of an organic photosensitive layer and E2 in the case of an inorganic photosensitive layer in FIG. 17.
In the conventional image forming apparatus 1 shown in FIG. 16, the amount of light radiated by the optical device 5 cannot be adjusted in accordance with the material of the photosensitive layer 2. Accordingly, the image density on the recording paper sheet 7 differs by the material of the photosensitive layer 2, resulting in reduction in the image quality. Further, it is troublesome to adjust the amount of light from the optical device 5 for each image forming apparatus.